Multiple data streams (for networks with multiple roots)

[matham]

Part of the accommodations made for multiple data streams (for networks with multiple roots). This should probably be clarified, but a user may pass a list of tensors (samples X features) where the i-th root receives the i-th external data tensor. Could be done with 3D tensors too, of course, but I'm sure something biased my decision when I made it.

Originally posted by @rm875 in #1 (comment)

As we talked, it maybe better if it was a dict somehow, instead of an indexed list.